I would say day 3 or so, but you will want to supervise and they probably won't swim for long. Make sure they can dry off really quick and stay warm. Have fun!
mine are a little over a week and I fill the kitchen sink with warm water just a couple inches and they are only in there for a few minutes. I use a big towel dry them off good and put them back under the brooder light
I vote a couple of days with supervision. They don't develop their oil gland until they are a week or so old and some people like to wait until they are producing their own oils.